A major problem in the field of optical data processing is the limited number of matched filters that can be stored and addressed. A Vander Lugt matched filter is a holographic representation of the Fourier transform of an object stored on a photographic plate. A correlator uses this matched filter to identify and locate a desired object in some arbitrary input image. In its basic configuration, the Vander Lugt correlator requires that an input scene of interest be presented on a collimated, coherent beam of light. When the light passes into the correlator it passes through a lens and falls on a matched filter, which is accurately positioned in the back focal plane of the lens. Careful alignment of the matched filter and the focused beam is of critical importance. The lens is commonly referred to as the Fourier transform lens and its back focal plane as the Fourier transform plane. It can be shown that the light amplitude in the Fourier transform plane is mathematically the Fourier transform of the amplitude distribution of the input image impressed on the coherent light beam.
Two prior art solutions have been presented to the problem of storing many matched filters. In one of these methods, multiple holograms of the Fourier transforms of reference objects are stored at a single location on a photographic plate by multiple exposure. The capacity of the plate to store matched filters is limited however and, typically, the maximum number is about 8. In the second method the Fourier transform lens is replaced by a holographic optical element which generates an array of Fourier transforms at distinct points in the Fourier transform plane. The currently achieved limit this way has been 25 Fourier transforms. By combining both methods a total of 75 simultaneously addressed matched filters has been achieved. Although some improvement in this number can be anticipated, a ten fold increase is not likely. Typical of the problems encountered with the prior art methods, is the limited number of exposures on matched filters (8 and 25) that may be obtained. The holographic lenses allow arrays of matched filters to be made, but they also introduce a noise factor which greatly reduces the signal-to-noise ratio of the correlation signal. In addition, the holographic lenses tend to be very lossy and the small fraction of the incident light which is Fourier transformed is spread out over a large number of matched filters simultaneously. The overall efficiency of this method is quite low.